add function argument name to substring and substr

hi.

attached patch add function argument name to function substring and substr
you can see the visual changes.
mater behavior:

\df substring
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+-----------+------------------+-------------------------+------
pg_catalog | substring | bit | bit, integer | func
pg_catalog | substring | bit | bit, integer, integer | func
pg_catalog | substring | bytea | bytea, integer | func
pg_catalog | substring | bytea | bytea, integer, integer | func
pg_catalog | substring | text | text, integer | func
pg_catalog | substring | text | text, integer, integer | func
pg_catalog | substring | text | text, text | func
pg_catalog | substring | text | text, text, text | func

with patch
List of functions
Schema | Name | Result data type | Argument
data types | Type
------------+-----------+------------------+--------------------------------------------------+------
pg_catalog | substring | bit | bits bit, start integer
| func
pg_catalog | substring | bit | bits bit, start integer,
count integer | func
pg_catalog | substring | bytea | bytes bytea, start integer
| func
pg_catalog | substring | bytea | bytes bytea, start integer,
count integer | func
pg_catalog | substring | text | string text, pattern text
| func
pg_catalog | substring | text | string text, pattern
text, escape_character text | func
pg_catalog | substring | text | string text, start
integer | func
pg_catalog | substring | text | string text, start
integer, count integer | func

I did the same change to the function substr.
since 9.7.2. SIMILAR TO Regular Expressions we use
substring(string, pattern, escape-character)
so i refactor the substring function argument name to
substring(string text, pattern text, escape_character text).

we can make it as ``substring(string text, pattern text, escape text).``
then in 9.7.2, we need to change the substring synopsis section.

one thing I am not sure is about
9.4. String Functions and Operators
Table 9.9. SQL String Functions and Operators.
Do we need entries for substring related functions?

current signature
substring ( string text [ FROM start integer ] [ FOR count integer ] )
is kind of different from
substring(string text, start integer, count integer).
since the previous one can allow keywords "FROM", "FOR", the latter one won't.